Nakon tuša u kupaonici ostane vlaga na ogledalu, pa ga moram obrisati.

Because nakon requires the genitive case.

• base form: tuš = shower
• genitive singular: tuša

So nakon tuša means after a shower / after showering.

This is a very common pattern:

• nakon posla = after work
• nakon ručka = after lunch
• nakon filma = after the film

In this sentence, it is most naturally understood as the act of taking a shower.

So nakon tuša is basically after showering or after a shower.

Croatian often uses a noun this way where English might prefer a verb phrase.

Because u can mean either:

• in/inside a place → then it takes the locative
• into a place → then it takes the accusative

Here there is no movement. It means in the bathroom, so Croatian uses the locative:

• kupaonica → u kupaonici

Compare:

• u kupaonici = in the bathroom
• u kupaonicu = into the bathroom

This is mainly a question of aspect.

• ostaje is imperfective
• ostane is perfective

In this sentence, ostane focuses on the result: after the shower, moisture ends up being left on the mirror.

A rough nuance is:

• ostaje vlaga = moisture remains / keeps remaining
• ostane vlaga = moisture gets left there / there ends up being moisture there

In everyday Croatian, ostane sounds very natural in this kind of repeated-result statement: after X happens, Y is left behind.

Vlaga is in the nominative singular because it is the subject of ostane.

So the structure is:

• vlaga ostane = moisture remains / moisture is left

The verb is in 3rd person singular because vlaga is singular.

Here vlaga means moisture, dampness, or the condensation left by steam after a shower.

It is a general, uncountable noun, so Croatian uses it in the singular.

In this context, English might naturally say steam or condensation, but vlaga is a normal Croatian choice.

Because na also changes case depending on meaning:

• location → locative
• movement onto something → accusative

Here the moisture is on the mirror, so this is location, and Croatian uses the locative:

• ogledalo → na ogledalu

Compare:

• na ogledalu = on the mirror
• staviti nešto na ogledalo = to put something onto the mirror

Here pa means something like:

• so
• and so
• therefore
• then

It links the first clause to its consequence:

• moisture stays on the mirror, so I have to wipe it

It is very common in everyday Croatian and often sounds more natural and conversational than a more formal connector.

Ga refers to ogledalo = the mirror.

So:

• pa ga moram obrisati = so I have to wipe it

The speaker is wiping the mirror, not the moisture itself.

Because the short unstressed pronoun ga can refer to a masculine or neuter singular object.

So even though ogledalo is neuter, ga is still correct.

The full stressed form would be njega, but in a normal sentence Croatian usually prefers the clitic ga.

Because ga is an unstressed clitic pronoun, and Croatian usually places clitics near the beginning of the clause.

So pa ga moram obrisati is the natural order.

English speakers often expect something more like I have to wipe it, where the pronoun comes later, but Croatian clitics behave differently and tend to appear early.

Because morati = must / have to is followed by an infinitive.

So:

• moram obrisati = I have to wipe

Also, obrisati is perfective, which fits well here because the meaning is wipe it off / wipe it clean, as a completed action.

Compare:

• brisati = to be wiping / to wipe in general
• obrisati = to wipe off completely

Because Croatian often omits subject pronouns when the verb ending already shows who the subject is.

• moram already means I have to

So ja is unnecessary unless you want emphasis:

• Ja moram obrisati = I have to wipe it

Without emphasis, Croatian normally just says moram.